1. Field of the Invention
The present invention relates to a projection television apparatus for enlarging and projecting on a large-size screen an image formed on the fluorescent screen of a projection cathode-ray tube and more particularly to a projection television apparatus capable of projecting a high contrast image.
2. Description of the Prior Art
In order to obtain a large-size television image, there has been devised and demonstrated a system in which an image formed on the fluorescent screen of a projection cathode-ray tube is enlarged and projected on a large-size screen by means of a projection lens. In this system, since it is desired that the image projected on the screen be as bright as possible, it is required that the projection cathode-ray tube produce an intensely bright image and it is necessary to employ a bright projection lens.
In order to produce an intensely bright image, there has been devised and demonstrated a liquid-cooled projection cathode-ray tube as shown in FIG. 1. In this liquid-cooled projection cathode-ray tube, a metal plate 3 with a center aperture is securely bonded with an adhesive 4 to the faceplate 2 of a projection cathode-ray tube 1, and a glass plate 5 is securely bonded with an adhesive 6 over the metal plate 3. A transparent liquid 7 which has good heat convection properties is sealed into a closed space defined by the faceplate 2, the metal plate 3 and the glass plate 5. The heat transmitted from the faceplate 2 is further transmitted to the metal plate 3 by the convection of the transparent liquid 7 and is dissipated into the surrounding atmosphere from the surface of the metal plate 3, whereby the faceplate 2 is cooled. As a result, even when the driving power is increased, the liquid-cooled projection cathode-ray tube can avoid likelihood of explosion and produce an intensely bright image.
On the other hand, there have been known bright projection lenses with Fl.0 consisting of six glass lens elements. Also, U.S. Pat. Nos. 4,300,817 and 4,348,081 disclose a projection lens which consists of three lens elements made of acrylic resin and which has an aspheric surface. When the lens elements are all made of plastic, the image plane is shifted due to variation in ambient temperature because the temperature dependence of plastic lenses is ten times larger than that of glass lenses. As a result, the image is out of focus, resulting in a convergence error. In order to overcome the above described problem, a hybrid lens consisting of three lens elements made of glass and plastic is disclosed in Japanese Laid-Open Patent Application No. 58-125007. Furthermore, Japanese Laid-Open Patent Application No. 58-181009 discloses a projection lens system which is combined with a projection cathode-ray tube whose luminescent screen is convex so as to make easy the correction of the image plane distortion, to improve aberrations, and to increase the relative aperture and the view angle.
As described above, the performance of the projection cathode-ray tube and the projection lens system have been improved, but there still remains the serious problem that the contrast of the projected image is not satisfactory. To overcome this problem, Japanese Laid-Open Patent Application No. 58-194234 disclosed a system as shown in FIG. 2. In this system, a transparent medium 12 whose index of refraction is substantially equal to those of the faceplate 9 of a projection cathode-ray tube 8 and the lens element 11 of a projection lens system 10 adjacent to the faceplate 9 is interposed between the faceplate 9 and the lens element 11, whereby contrast is improved. The projection lens system 10 is substantially similar in construction to the above-described projection lens consisting of three lens elements. The lens element 11 is therefore a planoconcave lens element whose concave surface is oriented toward the screen.
When the outer surface 13 of the faceplate 9 is flat and in contact with the air, a part of the light emitted from a point on a fluorescent screen 14 is reflected by the outer surface 13 and returns to the fluorescent screen 14 at the same angle as the angle at which the light is incident on the outer surface 13. The fluorescent screen 14 is substantially a complete diffusion reflection surface so that the light returned to the luminscent screen 14 is diffused and reflected to directed as unwanted light toward the projection lens system 10. The unwanted light is spread all over the screen, resulting in the decrease in contrast of the whole projected image. In the system as shown in FIG. 2, since the lens element 11 is a planoconcave lens whose concave surface is oriented toward the screen and the difference in index of refraction at the boundaries between the fluorescent screen 14 and the concave surface 15 is very little, the outer surface of the faceplate may be regarded as a concave surface. In this system, the light emitted from a point on the fluorescent screen 14 is diverged by the concave surface 15. In this case, the angle of divergence becomes greater than that prior to the reflection so that the illuminance of the light returned to the fluorescent screen 14 becomes smaller as compared with the case in which the outer surface 13 of the faceplate is in direct contact with the air. As a result, the contrast of the projected image is considerably improved as compared with the prior art projection television systems.
It is now apparent that if the liquid-cooled projection cathode-ray tube as shown in FIG. 1 and the projection lens system as shown in FIG. 2 and capable of improving contrast are combined, there may be provided a projection television apparatus which can project on a large screen an intensely bright image with a high degree of contrast. In the case of the recent mainstream projection television apparatus or receivers, red, green and blue optical systems are used to produce a color image on a screen. As a result, the arrangement as shown in FIG. 3 is used to project an intensely bright image with a high degree of contrast. Projection cathode-ray tubes 23, 24, 25 and projection lens systems 16, 17 and 18 are arranged in line so that the optical axes 19, 20 and 21 of the projection lens systems 16, 17 and 18 converge at one point on a screen 22. The optical axis 26 of the center green-image projection cathode-ray tube 24 can be aligned with the optical axis 20 of the projection lens system 17, but the axes 27 and 28 of the outer red-image and blue-image projection cathode-ray tubes 23 and 25 must be inclined at an angle with respect to the axes of the projection lens systems 16 and 18, respectively. The reason is that the red and blue images are projected at an angle on the screen 22 so that the deflection adjustments of the red and blue optical systems are required. Therefore the angle between the optical axes 27 and 28 of the red-image and blue-image projection cathode-ray tubes 23 and 25 becomes greater than the angle between the optical axes 19 and 21 of the projection lens systems 16 and 18.
When the air is filled in space between the projection lens system and the projection cathode-ray tube, there arises no problem even when the optical axis of the projection cathode-ray tube is inclined with respect to the optical axis of the projection lens system. But when the projection lens system and the projection cathode-ray tube cannot be spaced apart from each other as shown in FIG. 3, there arises a very serious problem. In the arrangement as shown in FIG. 3, transparent media 29, 30 and 31 must be interposed between the projection lens systems 16, 17 and 18 on the one hand and the projection cathode-ray tubes 23, 23 and 25 on the other hand so as to eliminate the air layers therebetween. Furthermore the transparent media 29, 30 and 31 are different in shape from each other. It is considered that when a projection television receiver or apparatus is assembled, a transparent media is in the liquid phase, but when the receiver or apparatus is operated, it is gel or solid. The reason is that in order to eliminate the air layer completely in assembly, the transparent media must be in the liquid phase. As a consequence, there must be provided parts which define and maintain a predetermined positional relationship between the projection lens system and the projection cathode-ray tube and which also serve to seal a transparent medium therebetween. These parts are of course very expensive and at least two types of parts are required because both the surfaces of the part used in the green optical system are flat or in parallel with each other while the parts which are used in the red and blue optical systems have surfaces which are not in parallel with each other. Furthermore, it is difficult to fabricate a part whose surfaces are not in parallel with each other. In addition, it is difficult to assemble it. As a result, the arrangement as shown in FIG. 3 becomes inevitably expensive.